1. Field of the Invention
This invention relates to a ceramic guide pin which is used, for example, in a projection welding machine for welding small parts such as nuts to automobile bodies, panels, etc., and a process for producing the same.
2. Description of the Prior Art
As a method of attaching small parts, e.g. nuts, to an automobile body or the like, spot welding by a projection welding machine has been generally practiced.
In the method, the nut is first fitted into a tip portion of a guide pin projected from a central portion of a cylindrical electrode. The nut, thus fitted in the guide pin, and a panel (vehicle body) are clamped between upper and lower electrodes, whereby the nut is welded to the panel. In this case, the guide pin is electrically insulated to prevent arcing, which would otherwise occur between the guide pin and the nut with the result of depletion of material through melting.
Metallic guide pins ordinarily used for this purpose are insulated by an Al.sub.2 O.sub.3 coating formed thereon by an oxidizing treatment or flame spraying. Such metal guide pins are poor in wear resistance, because the surface layers of the guide pins have a low hardness, for example, a knoop hardness of the order of a few GPa. Therefore, the insulation film on the metal guide pins are liable to be exfoliated by friction, galling or the like during the nutguiding operations, resulting in insulation failure. Accordingly, such guide pins must be replaced after a relatively short time of use.
When the nut fitted into the guide pin is inclined, in addition, the nut will be pressed as it is, namely, while remaining in partial contact with the guide pin, because the guide surface of the metallic guide pin has a very coarse surface roughness of Rmax 30 to 35 .mu.m and poor slidability. Even though the guide pin may not snap, the pin portion in contact with the nut may be dented or the insulation film may be exfoliated due to galling by a corner portion of the nut, resulting in failure of insulation. This causes the nuts to be welded to the panel in the inclined condition or with deviations from the intended position, leading to frequent occurrence of defective nut welding. In such a situation, the guide pin becomes unusable, even if not snapped.
In view of the above, ceramic-made guide pins which, though brittle, have a high hardness, e.g. a knoop hardness of 14 to 16 GPa, together with excellent insulating properties and heat resistance, have been proposed recently.
The present inventors also have proposed, in Japanese Patent Laid-open (Kokai) No. 1-224175, a ceramic-made guide pin having an endurance (the number of use cycles endured) of not less than 50,000, which is several times higher than those of metal-made guide pins.
However, the ceramic guide pins proposed in the Japanese Patent Laid-open No. 1-224175, though superior to metallic guide pins in endurance by a factor of from 2 to 5, have not necessarily been profitable industrially, because the manufacturing cost of the ceramic guide pins is as high as about 25 times that of the metallic guide pins.
The major cause of the higher production cost of ceramic guide pins is the grinding step subsequent to sintering. Namely, the production of the ceramic guide pins requires a precision finish by a diamond grinding wheel, leading to a long processing time and low productivity, or poor mass-producibility.